Assign Iceberg field IDs recursively in list elements

[osipovartem]

Problem

new_fields_with_ids assigned an ID to a list element but not to fields inside a struct element. Iceberg table creation for ARRAY(OBJECT(v VARIANT)) then failed with a missing Parquet field ID.

Change

• Assign IDs recursively through list elements while keeping Variant storage fields unnumbered.
• Reserve existing IDs and skip them when allocating new IDs, avoiding duplicates.
• Return an error on ID exhaustion instead of overflowing; propagate the new Result<Fields, Error> API through the two fork planner callsites.

Validation

• Red/green regressions for list-of-struct, mixed existing IDs, i32::MAX, and exhaustion.
• cargo +1.95.0 test -p iceberg-rust-spec --lib: 181 passed, 1015 ignored.
• cargo +1.95.0 check -p datafusion_iceberg -p datafusion-iceberg-sql.
• cargo +1.95.0 clippy -p iceberg-rust-spec -p datafusion_iceberg -p datafusion-iceberg-sql --all-targets --all-features -- -D warnings; formatting check.
• Independent read-only review: approved.

Downstream callers of new_fields_with_ids must handle Result; Rustice callsites are being updated.